A study of the relationship between the Electromyographic (EMG) signal and the isometric force output of three human skeletal muscles will be performed on 40 healthy adults. The subjects will be organized in four categories each consisting of approximately 10 subjects: accomplished swimmers, accomplished weight lifters, skilled typists or organists and subjects not having any of the previous specializations. The possible dependence of the EMG-force relationship on functionally different muscles such as the first dorsal interosseous biceps brachii and deltoid, and on different forms of exercise will be investigated. The statistical and physiological properties of single motor units during varying force and force-rate contractions will be investigated and modelled mathematically according to random-processes analysis approaches that have proven successful for isometric constraint-force contractions. The model of the motor unit properties will be used to construct a model for the EMG signal as a function of the muscle force and force-rate. The purpose of this study is to a) provide a basis for relating the EMG signal to the force output of human skeletal muscles, and b) to describe some physiological properties of the motor units in normal muscles. This study will also provide a non-invasive technique and baseline data which can be used to quantitatively measure the difference between the performance of normal or abnormal muscles and thus should result in a useful clinical implement for evaluating the state of a muscle.